This invention relates to nonlinear resistors (varistors) comprising sintered disks of zinc oxide with other metal oxides. More specifically, this invention relates to improved nonlinear resistor materials having low breakdown voltages and leakage currents.
The construction and use of nonlinear resistor materials formed by the sintering of zinc oxide with other metal oxides are described in U.S. Pat. Nos. 3,682,841 to Matsuoka et al. and 3,687,871 to Masuyama which are incorporated by reference in this disclosure.
The electrical characteristics of varistor materials which are of primary importance to design engineers include: Breakdown Voltage (V.sub.B), Nonlinear Coefficient (.alpha.), Leakage Current (I.sub.L), Average Grain Size (d), and Barrier Voltage (v.sub.b). As used below the term "Breakdown Voltage (V.sub.B)" means the voltage gradient necessary to produce a current density of 10.sup..sup.-3 amps cm.sup..sup.-2. "Nonlinear Coefficient (.alpha.)" is defined by the formula ##EQU1## Where V.sub.1, V.sub.2 and I.sub.1, I.sub.2 respectively are the voltage gradient and corresponding current density values taken at points within the region of nonlinear resistor operation characterized by a power law voltage-current relationship EQU I = (V/C)
for the compositions discussed below, I.sub.1 is typically 10.sup..sup.-5 amp cm.sup..sup.-2 and I.sub.2 is 10.sup..sup.-3 amp cm.sup..sup.-2. "Leakage Current (I.sub.L)" is the current density at a voltage gradient equal to one half the breakdown voltage (V.sub.B). "Barrier Voltage (v.sub.b)" is a statistical parameter calculated by multiplying the breakdown voltage (V.sub.B) by the average zinc oxide grain size (d) as determined by microscopic examination.
Nonlinear electrical resistors presently offered for sale have nonlinear coefficients (.alpha.) of approximately 50 and breakdown voltage ratings on the order of 120 volts mm.sup..sup.-1. The structure of these ceramic materials was known to constitute an aggregation of zinc oxide crystals dispersed in an intergranular medium but specific details of the microstructure and conduction mechanism remained substantially unknown.
It is desirable, for the production of useful circuit components, to produce nonlinear resistor materials having breakdown voltage ratings lower than the 120 volts mm.sup..sup.-1 presently available. Previous attempts to produce these breakdown voltage ratings have, however, yielded materials having poorly defined breakdown characteristics, low nonlinear coefficient (.alpha.), and high values of leakage current. These materials have poorly defined breakdown voltage curves and must dissipate considerable power when operated below breakdown voltage. They are, therefore, generally unsuitable for use as transient and overvoltage suppressors, a major field of varistor application.